7094
J. Org. Chem. 1999, 64, 7094-7100
Convergent Solution-Phase Combinatorial Synthesis with Multiplication of Diversity through Rigid Biaryl and Diarylacetylene Couplings Dale L. Boger,* Weiqin Jiang, and Joel Goldberg Department of Chemistry and the Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037 Received April 16, 1999
The solution-phase synthesis of iminodiacetic acid diamide libraries linked to aryl iodides and their Pd-catalyzed dimerization are detailed. Mixtures containing 64 980 components are synthesized in only 4 steps from N-BOC-iminodiacetic acid anhydride (1) and 21 readily available starting materials, demonstrating the multiplication of diversity achievable by the convergent assembly of building blocks. Both biaryl formation and sequential Stille couplings with bis(tributylstannyl)acetylene are utilized to dimerize the functionalized iminodiacetic acid diamide precursors resulting in product libraries with two sets of binding groups separated by variable length rigid linkers suitable for probing protein-protein interactions. Deconvolution libraries synthesized alongside the full mixtures allow for identification of active components. Introduction Ligand-induced receptor and protein dimerization or oligomerization has emerged as a general mechanism for signal transduction1 and important members of many receptor superfamilies are activated by such a process.2-5 Many of these receptors and proteins appear to bind their ligands using only a small cluster of residues for a majority of the binding interaction.6,7 This has led to the expectation that small molecules may be capable of inducing their dimerization and activation, and recently peptide,7,8 as well as nonpeptide,9 agonists promoting receptor homodimerization have been identified through the random screening of compound libraries. Our interest in combinatorial chemistry rested on its potential to provide candidate leads for promoting receptor activation by dimerization. This, coupled with the potential of utilizing a single approach for the discovery of antagonists and their conversion to agonists, is an important element underlying our continued development10,11 of (1) Hinterding, K.; Alonso-Diaz, D.; Waldmann, H. Angew. Chem., Int. Ed. Engl. 1998, 37, 688. Klemm, J. D.; Schreiber, S. L.; Crabtree, G. R. Annu. Rev. Imunol. 1998, 16, 569. Heldin, C.-H. Cell 1995, 80, 213. (2) Ullrich, A.; Schlessinger, J. Cell 1990, 61, 203. (3) Moutoussamy, S.; Kelly, P. A.; Finidori, J. Eur. J. Biochem. 1998, 255, 1. (4) Massague, J.; Attisano, L.; Wrana, J. L. Trends Cell Biol. 1994, 4, 172. Lemmon, M. A.; Schlessinger, J. Trends Biochem. Sci. 1994, 19, 459. (5) Smith, C. A.; Farrah, T.; Goodwin, R. G. Cell 1994, 76, 959. (6) Wells, J. A. Science 1996, 273, 449. Reineke, U.; SchneiderMergener, J. Angew. Chem., Int. Ed. Engl. 1998, 37, 769. (7) Livnah, O.; Stura, E. A.; Johnson, D. L.; Middleton, S. A.; Mulcahy, L. S.; Wrighton, N. C.; Dower, W. J.; Jolliffe, L. K.; Wilson, I. A. Science 1996, 273, 464. (8) Cwirla, S. E.; Balasubramanian, P.; Duffin, D. J.; Wagstrom, C. R.; Gates, C. M.; Singer, S. C.; Davis, A. M.; Tansik, R. L.; Mattheakis, L. C.; Boytos, C. M.; Schatz, P. J.; Baccanari, D. P.; Wrighton, N. C.; Barrett, R. W.; Dower, W. J. Science, 1997, 276, 1696. Kimura, T.; Kaburaki, H.; Miyamoto, S.; Katayama, J.; Watanabe, Y. J. Biochem. 1997, 122, 1046, (9) Tian, S.-S.; Lamb, P.; King, A. G.; Miller, S. G.; Kessler, L.; Luengo, J. I.; Averill, L.; Johnson, R. K.; Gleason, J. G.; Pelus, L. M.; Dillon, S. B.; Rosen, J. Science 1998, 281, 257. Kimura, T.; Kaburaki, H.; Tsujino, T.; Ikeda, Y.; Kato, H.; Watanabe, Y. FEBS Lett. 1998, 428, 250.
solution-phase techniques (Figure 1).12 Recently we disclosed an example of the convergent combination of a small number of monomers that is especially suited for the discovery of receptor antagonists and their derivitization into potential receptor dimerization agonists.13 Mixture libraries containing 476 775 and 114 783 975 components were prepared by the dimerization of functionalized iminodiacetic acid diamides via the olefin metathesis reaction. Deconvolution of these mixtures can be accomplished by the complementary techniques of positional scanning14 and deletion synthesis deconvolution.13 Complementary to our development of the olefin metathesis reaction to simultaneously join iminodiacetic acid derivatives and randomize the length of the linking tether, we have explored other methods of such convergent library synthesis. Herein we report the palladiumcatalyzed dimerization of iodoarene-appended iminodiacetic acid diamide libraries (Figure 2). As in the olefin metathesis studies, these libraries were designed to contain binding groups of appreciable size to allow sufficient contact with target protein surfaces. In this investigation we explored dimerization methods which result in a rigid core from which the pendent binding groups are directed toward the target proteins. This may (10) Cheng, S.; Comer, D. D.; Williams, J. P.; Myers, P. L.; Boger, D. L. J. Am. Chem. Soc. 1996, 118, 2567. Boger, D. L.; Tarby, C. M.; Myers, P. L.; Caporale, L. H. J. Am. Chem. Soc. 1996, 118, 2109. Cheng, S.; Tarby, C. M.; Comer, D. D.; Williams, J. P.; Caporale, L. H.; Myers, P. L.; Boger, D. L. Bioorg. Med. Chem. 1996, 4, 727. Boger, D. L.; Ducray, P.; Chai, W.; Jiang, W.; Goldberg, J. Bioorg. Med. Chem. Lett. 1998, 8, 2339. Boger, D. L.; Ozer, R. S.; Andersson, C.-M. Bioorg. Med. Chem. Lett. 1997, 7, 1903. Boger, D. L.; Chai, W. Tetrahedron 1998, 54, 3955. Boger, D. L.; Chai, W.; Ozer, R. S.; Andersson, C.-M. Bioorg. Med. Chem. Lett. 1997, 7, 463. (11) Boger, D. L.; Goldberg, J.; Jiang, W.; Chai, W.; Ducray, P.; Lee, J. K.; Ozer, R. S.; Andersson, C.-M. Bioorg. Med. Chem. 1998, 6, 1347. (12) For a recent review on solution-phase combinatorial chemistry, see: Merritt, A. T. Comb. Chem. High Throughput Screening 1998, 1, 57. (13) Boger, D. L.; Chai, W.; Jing, Q. J. Am. Chem. Soc. 1998, 120, 7220. (14) Dooley, C. T.; Houghten, R. A. Life Sci. 1993, 52, 1509. Pinilla, C.; Appel, J. R.; Blanc, P.; Houghten, R. A. Biotechniques 1992, 13, 901.
10.1021/jo990639p CCC: $18.00 © 1999 American Chemical Society Published on Web 08/21/1999
Convergent Solution-Phase Combinatorial Synthesis
J. Org. Chem., Vol. 64, No. 19, 1999 7095 Scheme 1
Figure 1.
Figure 2. Dimerization via biaryl and diarylacetylene rigid linkages which position target binding groups R1 and R2.
be an especially important property for receptor dimerization agonist libraries considering recent studies which suggest receptor activation is achievable only by a precise (15) Livnah, O.; Stura, E. A.; Middleton, S. A.; Johnson, D. L.; Jolliffe, L. K.; Wilson, I. A. Science 1999, 283, 987. Remy, I.; Wilson, I. A.; Michnick, S. W. Science 1999, 283, 990. Ballinger, M. D.; Wells, J. A. Nature Struct. Biol. 1998, 5, 938. Livnah, O.; Johnson, D. L.; Stura, E.; Farrell, F. X.; Barbone, F. P.; You, Y.; Liu, K. D.; Goldsmith, M. A.; He, W.; Krause, C. D.; Pestka, S.; Jolliffe, L. K.; Wilson, I. A. Nature Struct. Biol. 1998, 5, 993. Syed, R.; Reid, S. W.; Li, C.; Cheetham, J. C.; Aoki, K. H.; Liu, N.; Zhan, H.; Osslund, T. D.; Chirino, A. J.; Zhang, J.; Finer-Moore, J.; Elliott, S.; Sitney, K.; Katz, B. A.; Matthews, D. J.; Wendoloski, J. J.; Egrie, J.; Stroud, R. M. Nature 1998, 395, 511.
(re)orientation of the membrane bound target proteins.15 Two palladium-catalyzed dimerization protocols are described which link functionalized iodoarene precursors, biaryl formation by direct homocoupling, and diarylacetylene formation by sequential Stille couplings with bis(tributylstannyl)acetylene. Developmental Studies. Initial efforts focused on the preparation of a modest library to validate the approach and allow for optimization of the reaction conditions (Scheme 1). N-BOC-iminodiacetic acid anhydride (1), formed in situ from the readily available dicarboxylic acid, was the starting point for the library synthesis.10,11 Functionalization with phenethylamine provided monoamide 2 in 90% yield, and subsequent coupling (PyBOP) of the remaining carboxylic acid with 4-methoxyphenethylamine provided 3 in 92% yield. Simple liquid-liquid extractions provided pure products for both steps. The third functionalization of the iminodiacetic acid template involved the coupling to an iodoarene suitable for Pdcatalyzed dimerization. Variability in the chain-length connecting the iminodiacetic acid template to the dimerization center was included to allow the binding groups to span a variety of distances. Both meta and para substitution of the iodobenzamides were also chosen to increase the diversity of reaction products. N-BOC deprotection of 3 (HCl-dioxane) and PyBrOP coupling of the crude HCl salt with the 10 iodoarene-linked carboxylic acids C1-C10 (Figure 3) provided the dimerization precursors 4-13 (48-84%). Regardless of the conversion, unreacted starting materials or reaction byproducts were easily removed by acidic and basic liquid-liquid extractions providing pure products. With the model iodoarene precursors in hand, their symmetrical and unsymmetrical dimerizations were investigated16 (Scheme 2). Initial experiments with the Stille coupling reaction between the iodoarenes (4-13)
7096 J. Org. Chem., Vol. 64, No. 19, 1999
Boger et al. Scheme 2
Figure 3. The 21 building blocks used to assemble both the biaryl and diarylacetylene libraries.
and bis(tributylstannyl)acetylene17 suggested that the reactivities of the iodobenzamide derivatives were nearly identical regardless of the amide substitution (chain length) or site of substitution (meta or para). Optimum conditions were found to be reaction with bis(tributylstannyl)acetylene (0.5 equiv) and Pd(Ph3)4 (0.05 equiv) in dioxane at 100 °C for 4 h. Addition of a few crystals of BHT were found to improve the reaction yields which ranged from 40 to 68% for the homodimerizations 4-13 f 14-23. Other additives such as LiCl17 used in similar couplings were found to have no effect on these reactions. The iodides were completely consumed under these conditions, and no reduction or other byproducts (e.g., biaryl formation) were observable. The lack of these byproducts from 14 and 19 was further demonstrated by an independent synthesis of the biaryl products 25 and 30 and the reduction product 36 (Scheme 3). Comparison of these products by 1H NMR spectroscopy and TLC to that of the crude Stille product confirmed their absence. Consequently, purification mainly involved the removal of catalyst and tin byproducts. This was accomplished (16) The corresponding aryl bromides were also tested and found to be either too unreactive or lead to unacceptable amounts of reduction byproduct in both the Stille and biaryl coupling reactions. (17) For the use of bis(tributylstannyl)acetylene to synthesize diarylalkynes, see: Cummins, C. H. Tetrahedron Lett. 1994, 35, 857.
Scheme 3
by eluting the reaction mixtures from a short column of SiO2. All impurities were found to be removable by this method, providing compounds judged clean by NMR spectroscopy. Unsymmetrical couplings were first examined by reacting an equimolar mixture of the 5 m-iodobenzamide
Convergent Solution-Phase Combinatorial Synthesis
Figure 4. ESMS of the dimerization sublibraries synthesized from the couplings of a mixture of 4 and 6-9 by either (a) Stille coupling with bis(tributylstannyl)acetylene, Pd(Ph3)4 or (b) homocoupling with 10% Pd/C, Et3N. Each product mixture consists of 15 components with 14 distinct molecular weights.
derivatives 4 and 6-9 in a single reaction vessel using the same reaction and purification conditions optimized for the individual compounds. The purified product mixture contained all 15 dimeric products18 (63% yield) as confirmed by MS analysis of the product mixture (Figure 4a). Similarly a mixture of the p-iodobenzamide derivatives 10-13 were reacted providing the expected mixture of 9 products in comparable yield. The meta and para mixture couplings were investigated by reacting 7 and 12 in which a statistical 1:1:2 distribution of the two homo- and hetero-coupled products (16, 22, and 24) were observed, the mixture yield being 42%. We have previously reported the synthesis of biaryl libraries by the Pd/C-catalyzed homocoupling of substituted iodoarenes.19 This was extended to the same iodoarene-linked iminodiacetic acid diamides used in the Stille coupling reactions. The test reactions for biaryl (18) A statistical dimerization of n components results in a mixture of n(n + 1)/2 products. (19) Boger, D. L.; Goldberg, J.; Andersson, C.-M. J. Org. Chem. 1999, 64, 2422.
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synthesis (individual homocouplings of 4-13, Scheme 2) worked effectively with the best conditions being 10% Pd/C (0.1 equiv) and Et3N (2.0 equiv) in DMF at 100 °C, for 18 h. All starting materials were consumed, and the product biaryls were isolated in moderate to good yields for both the meta- and para-substituted iodobenzamides. A small amount of reduction byproduct (e.g., 36) was observed even under these optimized conditions; however, conducting the reactions highly concentrated (0.2 M) kept this to a minimum (
